This invention relates to compositions for treating contact lenses, especially rigid, gas permeable contact lenses.
The surfaces of contact lenses must have a certain degree of hydrophilicity to be wet by tears. Tear wettability is in turn necessary to provide the lens wearer with comfort and good vision.
One way to impart wettability to contact lens surfaces is to add hydrophilic monomers to the mixture of comonomers used to form the contact lens material. However, the relative amount of hydrophilic monomer added affects physical properties other than wettability. For example, the hydrophilic monomer content of rigid gas permeable lens materials is much less than that of soft, hydrogel lenses. The rigid lenses accordingly contain only a few percent water of hydration whereas soft lenses contain amounts varying from 10 to 90%. Thus, while hydrophilic monomer addition does increase wettability, the technique is limited by the influence that it has on other properties.
Another way to impart wettability to lens surfaces is to modify the surface after polymerization. For example, surface coatings of hydrophilic polymers have been grafted onto the surface. Plasma treatment has also been used to increase the hydrophilicity of hydrophobic surfaces. Although effective, methods such as these are often expensive (requiring complicated and difficult manufacturing procedures) and impermanent.
Water soluble polymers in lens care solutions have also been used to enhance the wettability of lens surfaces. Use of wetting polymers in this way provides a "cushion" between the lens and the eye which is equated with increased wettability as wearer comfort and tolerance. However, a common drawback with this approach is that the cushion layer dissipates rapidly, since there is little specific interaction between the polymer and the lens surface.
U.S. Pat. Nos. 4,168,112 and 4,321,261 disclose a method to overcome this drawback by immersing the lens in a solution of an oppositely charged ionic polymer to form a thin polyelectrolyte complex on the lens surface. The complex increases the hydrophilic character of the surface for a greater period of time relative to an untreated surface. Of particular interest are cellulosic polymers bearing a cationic charge, said polymers forming a strongly adhered hydrophilic layer on the contact lens surface. These polymers have proven to be exceptional components for wetting, soaking, and lubricating solutions.
Cationic surfactants greatly lower the surface tension of water and will accumulate on surfaces which have hydrophobic character. However, cationic surfactants are often not biocompatible with the eye. Some (i.e., benzalkonium chloride) are known to cause severe ocular reactions.